1. Field of the Invention
The invention is directed to an arrangement for fixing or securing the capstan on the shaft stub of a drive motor for a magnetic tape recorder means wherein this shaft stub is composed of a ceramic material.
2. Description of the Prior Art
In commercial magnetic tape recorder means which are utilized as digital stores in data-processing systems, the magnetic tape is frequently transported by only a single capstan. Take-up reel and take-off reel have drives which are separate therefrom and which are controlled via the loop length of the tape section buffered in buffer chambers. These magnetic tape recorder means are utilized in start/stop operation, the binary data are stored or, respectively, read out formated in blocks, and short interblock gaps of only a few millimeters are left between the blocks for reasons of storage capacity. At the same time, the tape speed in fast-running magnetic tape recorder means amounts, for example, to 200 ips, this corresponding to a tape speed of more than 5 m/s in the metric system.
It can be seen without further ado from these boundary conditions that extremely high demands are made of the drive system for the magnetic tape, this being composed of tape drive motor, capstan and of a tape speed control. The capstan seated on the shaft stub of a tape drive motor is therefore executed in lightweight fashion because of the mass moment of inertia and is essentially composed of a thin-walled, cylindrical cage in whose generated surface a plurality of passages are provided and which comprises a bearing sleeve in axial direction for the acceptance of the shaft stub of the tape drive motor. A light metal alloy, frequently a magnesium-aluminum alloy, is used for manufacturing this cage. Likewise in adaptation to these boundary conditions, a sintered material is utilized as the material for the motor shaft of the tape drive motor, this, given lowest weight, exhibiting great stiffness in comparison to other materials usually employed. This sintered material is preferably composed of an aluminum oxide ceramic. On the basis of this material, one succeeds in displacing torsional vibrations of the motor shaft into a frequency range which is uncritical for the control system of the tape drive.
However, this material selection has certain disadvantages, because the coefficients of thermal expansion of the materials for the capstan or, respectively, the motor shaft differ extremely. Difficulties in fixing or securing the capstan to the shaft stub of the tape drive motor result therefrom. Moreover, the ceramic material employed for the manufacture of the motor shaft is critical in terms of production engineering; one therefore attempts to suppress modifications in the diameter in the case of the motor shaft insofar as possible.
This structural principle then also has effects on the type of fastening the capstan to the shaft stub. Usually, a strike sleeve is slipped onto the shaft stub of uniform diameter and is glued to the shaft stub. A strike face against which the capstan is pushed during assembly is thus formed. The guide bore in the bearing sleeve of the capstan is toleranced with tight play in comparison to the outside diameter of the shaft stub and is therefore seated with precise fit. The capstan is fixed with a fastening screw which is screwed into the head end of the shaft stub in axial direction and which axially tenses the bearing sleeve of the capstan. The fastening screw is thereby seated in a threaded bush which is centrally let into the head of the shaft stub.
This conventional solution is definitely uncritical given normal operation of the magnetic tape recorder means. However, experience has shown that the pairing of materials can nonetheless lead to sporadic outages. Investigations of the outages have shown that these are to be attributed to ungovernable temperature influences during shipment from the manufacture to the place the apparatus is utilized. Temperatures that are far higher than usually found, for example, in a computer center or, respectively, than are reached during operation of the magnetic tape recorder means under normal conditions can be encountered on this shipment route. Given adequately high temperatures, such high axial forces already arise due to the different coefficients of thermal expansion of the materials for the motor shaft and for the bearing sleeve of the capstan that the glued location between strike sleeve and shaft stub yields and the strike sleeve yields slightly axially. Under normal operating conditions, the axial retaining forces for the capstan are no longer adequate for a faultless, slip-free running.
Also added thereto is that the reduced pressing power of the fastening screw promotes a loosening of this screw when the capstan runs in right-handed rotation. Damage to the tape drive can be the result.